1. Field of the Invention
This invention relates to aqueous polyurethane (PUR) dispersions having improved water and solvent resistance and to their use for coating substrates, in particular wood substrates.
2. Description of the Prior Art
Aqueous dispersions of polyurethanes or polyurethane foamed materials, which are described as PUR dispersions for simplicity, are known (see Angewandte Chemie, 82, (1970), pages 53 to 63, for example), and exhibit a high level of quality. They are used on a large scale for lacquers and coatings and fulfil stringent demands for many applications.
Nevertheless, there are applications where lacquers and coatings produced from PUR dispersions do not satisfy all of the necessary requirements, e.g., the coating of wooden parts and parquet floors. Very stringent demands are imposed on surface hardness, water resistance and resistance to mixtures of water and ethanol. Another important requirement is that curing, i.e., the formation of a coating, has to take place at a relatively low temperature, generally at ambient temperature, so that the additional crosslinking obtained by curing at elevated temperature cannot occur. When applied to wooden substrates and cured at ambient temperature, prior art PUR dispersions generally form films which are insufficiently resistant to water and water/ethanol mixtures.
An object of the present invention is to provide aqueous PUR dispersions which form films or coatings having improved resistance to water and water/ethanol mixtures.
This object may be achieved by using polyisocyanates containing allophanate groups for the production of aqueous PUR dispersions.
Many literature references include polyisocyanates containing allophanate groups in lengthy lists of polyisocyanates that are suitable for preparing PUR dispersions, but these polyisocyanates have not been described in detail and no examples demonstrating their use are known.